An optical fiber is manufactured by downwardly moving a quartz-based optical fiber glass preform (hereinafter called a glass preform) from an upper end opening of an optical fiber drawing furnace (hereinafter called a drawing furnace) to the inside of a furnace core tube and also heating and melting the distal end of the glass preform and decreasing the diameter of the distal end of this glass preform and being drawn from a lower end opening. Since the temperature of the inside of the drawing furnace at this time becomes very high (about 2000° C.), carbon with good heat resistance is used in a component of the inside of the drawing furnace.
This carbon has properties oxidized and consumed in a high-temperature oxygen-containing atmosphere. Because of this, it is necessary to hold the inside of the drawing furnace in an atmosphere of nitrogen gas, rare gas such as argon gas or helium gas (hereinafter called an inert gas etc.).
In this case, the outside air (oxygen) is prevented from entering the inside of the drawing furnace by setting the inside of the drawing furnace at positive pressure, but when airtightness is not obtained well in a gap between the glass preform and the upper end opening of the drawing furnace (the gap is not sealed), the outside air is sucked inside the drawing furnace to affect the life of the drawing furnace and also, the amount of use of the inert gas etc. is increased and a manufacturing cost of the optical fiber cannot be decreased. Hence, a seal mechanism for closing the gap between the glass preform and the upper end opening of the drawing furnace is required.
For example, Patent Reference 1 discloses a seal structure including two-step vertical blade members brought into contact with a side surface of a glass preform in an upper end opening of a drawing furnace.